1. Field of the Invention
The invention relates to labeled compounds for detection by infrared spectroscopy and in particular to compounds labeled with cyanate or thiocyanate metal complexes that are detectable in the energy region of 2300-1900 cm.sup.-1.
2. Description of the Related Art
Labeled compounds are used in assays for the detection and quantitation of molecular species. Types of labels that have been used include radioisotopes, enzymes, and fluorescent and phosphorescent compounds. Each of these types of labels has disadvantages. For example, radioisotopes have a limited half-life, and special precautions must be taken in their storage, use and disposal. Enzyme labels can be bulky and can interfere with binding reactions in assays. Moreover, enzymes are susceptible to denaturation and inhibition. Fluorescent labels may be prone to photobleaching or to fluorescence quenching if used near a metal or semiconductor surface. In addition, the fluorescent signal from a labeling molecule may be subject to interference due to fluorescence by the sample or by parts of the matrix or environment surrounding the sample. Moreover, commonly used fluorescent molecules are often difficult to attach to analytes, and, because they are often large molecules, they may change the properties of the analyte.
Recently, molecules that absorb energy in the infrared region of the spectrum have been developed as labels. Metal carbonyl complexes have been used as infrared-active dyes, and a technique called carbonylmetalloimmunoassay (CMIA) has been developed for using the carbonyl dyes in immunoassays. See, for example, U.S. Pat. No. 5,578,499 to Ismail, U.S. Pat. No. 4,656,142 to Jaouen et al, U.S. Pat. No. 4,983,646 to Jaouen et al, V. Philomin et al, "New Applications of Carbonylmetalloimmunoassay (CMIA): a Non-Radioisotopic Approach to Cortisol Assay" Journal of Immunological Methods, 171 (1994) pp 201-210, Samain et al "Carbonylmetalloimmunoassay (CMIA) a New Type of Non-Radioisotopic Immunoassay" Journal of Immunological Methods, 148, (1992) pp 65-75, Salmain et al "Fourier Transform Infrared Spectroscopic Method for the Quantitative Trace Analysis of Transition-Metal Carbonyl-Labeled Bioligands" Analytical Chemistry, vol 63, No. 20, Oct. 15, 1991, Wang et al, "Metal Carbonyl Labels for Oligonucleotide Analysis by Fourier Transform Infrared Spectroscopy", J. Am. Chem. Soc. 1993, 115, pp 4399-4400, Salmain et al "Use of Fourier Transform Infrared Spectroscopy for the Simultaneous Quantitative Detection of Metal Carbonyl Tracers Suitable for Multilabel Immunoassays" Analytical Biochemistry 208, (1993) pp 117-120 (all of the above patents and articles are incorporated herein by reference). Metal carbonyls have the property that they absorb energy very intensely in the region of 2000 to 1800 cm.sup.-1, a region of the spectrum that is relatively free of interferences from biological molecules and absorbances of H.sub.2 O. However, metal carbonyls are unstable and tend to decompose quickly, especially when exposed to air or moisture.